Generally the display panels of equipment like a laptop computer comprises some form of back light and in front of this is a liquid crystal display. The backlight may be an electroluminescent panel or a number of cold cathode fluorescent tubes with a suitable diffuser to ensure even lighting. Generally speaking there are problems with electroluminescent panels in that their long term reliability is not good and so the use of cold cathode fluorescent lamps is preferred.
A problem however with such lamps and their supply circuits is that they are relatively inefficient. For example, in a notebook computer with a black and white display the total energy required to drive the backlight amounts to something of the order of one third to one half of the total energy consumption. In a notebook computer with a colour display this proportion can be one half to two thirds of the total energy consumption. It is therefore highly desirable provide an invertor circuit for such cold cathode fluorescent lamps which is very efficient in terms of its energy consumption whilst still remaining small and compact in size. In that connection efficiency is also of considerable importance since portable equipment of this type usually has the option of drawing its power either from a mains supply or from battery power. If the efficiency of illumination is not high when the device is operating on battery power, then any wastage of power reduces the overall operating time on the battery power before re-charging is necessary.
Generally the invertor circuit used to supply a backlight needs to be positioned close to the display to minimise the length of high frequency leads which need mechanical protection and safety insulation to reduce high frequency radiation and to reduce risks of electric shock. In portable laptop computers therefore the invertor circuit must usually be positioned in the hinged display.
Cold cathode fluorescent lamps as used in this context generally require a high striking voltage, e.g. 1400 V peak, to ionize the gases in the lamp and so turn it on. Before the lamp is struck it has a high impedance because it is essentially an open circuit but, once it strikes, its resistance reduces to a low figure and it needs a lower voltage to run it. To ensure that the supply circuit is not short circuited it is generally necessary to include some form of ballasting reactance such as capacitor or inductor to limit the operating current and the resulting voltage drop across the reactance reduces the voltage actually applied across the lamp to its normal running voltage which tends to be of the order of 300 to 400 V rms.
To ensure a reasonably high overall efficiency it is desirable to operate such supply circuits at high frequency, e.g. 20 to 60 KHz. That usually represents the best overall compromise since the efficacy of fluorescent lamps increases asymptotically to approach a maximum at higher frequencies whilst efficiency of the driving circuit decreases with increase in frequency. However, conventional electronic ballast circuits for this sort of frequency supply generally need a relatively large sized transformer with a large core and/or a large number of turns. It is usually the size of this transformer which limits the overall thickness of the invertor circuit when fitted into the hinged display of a portable laptop computer.
The invention therefore aims to address these problems and to provide a supply circuit for any form of fluorescent lamp which is of improved efficiency and of compact size.